Probably for the same reason you can't ask in a Target whether the Wal-mart up the street has it in your size - the individual vendors are incentivized to avoid

Probably for the same reason you can't ask in a Target whether the Wal-mart up the street has it in your size - the individual vendors are incentivized to avoid you going to another source, and hope you'll decide to go with something "good enough" rather than paying the cost of going to another place and comparing things. That cost is much lower outside of brick and mortar travel times, but not 0, especially for people on crap internet connections or slow computers, and people tend to just want to get it done rather than shopping through N sites for the best option, so they pick "good enough". That, plus the vendors don't want you to think they're just an Amazon frontend so they have their own UI, plus if they allow suppliers to populate the item fields, good luck getting the suppliers to provide consistent results...